1. Field of the Invention
The present invention relates to pulse oximeter techniques for deriving cardiac and breathing parameters of a subject from extra-thoracic blood flow measurements, in particular the invention relates to medical devices and techniques for deriving breath rate, breath distention, and pulse distention measurements of a subject from a pulse oximeter system coupled to the subject.
2. Background Information
As background, one type of non-invasive physiologic sensor is a pulse monitor, also called a photoplethysmograph, which typically incorporates an incandescent lamp or light emitting diode (LED) to trans-illuminate an area of the subject, e.g. an appendage, which contains a sufficient amount of blood. FIG. 1 schematically illustrates the photoplethysmographic phenomenon. The light from the light source 10 disperses throughout the appendage, which is broken down in FIG. 1 into non-arterial blood components 12, non-pulsatile arterial blood 14 and pulsatile blood 16, and a light detector 18, such as a photodiode, is placed on the opposite side of the appendage to record the received light. Due to the absorption of light by the appendage's tissues and blood 12, 14 and 16, the intensity of light received by the photodiode 18 is less than the intensity of light transmitted by the LED 10. Of the light that is received, only a small portion (that effected by pulsatile arterial blood 16), usually only about two percent of the light received, behaves in a pulsatile fashion. The beating heart of the subject, and the breathing of the subject as discussed below, creates this pulsatile behavior. The “pulsatile portion light” is the signal of interest and is shown at 20, and effectively forms the photoplethysmograph. The absorption described above can be conceptualized as AC and DC components. The arterial vessels change in size with the beating of the heart and the breathing of the patient. The change in arterial vessel size causes the path length of light to change from dmin to dmax. This change in path length produces the AC signal 20 on the photo-detector, IL to IH. The AC Signal 20 is, therefore, also known as the photo-plethysmograph.
The absorption of certain wavelengths of light is also related to oxygen saturation levels of the hemoglobin in the blood transfusing the illuminated tissue. In a similar manner to the pulse monitoring, the variation in the light absorption caused by the change in oxygen saturation of the blood allows for the sensors to provide a direct measurement of arterial oxygen saturation, and when used in this context the devices are known as oximeters. The use of such sensors for both pulse monitoring and oxygenation monitoring is known and in such typical uses the devices are often referred to as pulse oximeters. These devices are well known for use in humans and large mammals and are described in U.S. Pat. Nos. 4,621,643; 4,700,708 and 4,830,014 which are incorporated herein by reference.
Current commercial pulse oximeters do not have the capability to measure breath rate or other breathing-related parameters other than blood oxygenation. An indirect (i.e. not positioned within the airway or airstream of the subject), non-invasive method for measuring breath rate is with impedance belts.
It is well established that it is critical to properly control anesthesia levels of a patient, or subject. In dealing with non-human subjects in animal research applications, having specialized anesthesiologists or specialized equipment is simply not an option for researchers. The use of breath-related parameters and heart-related parameters from easily applied non-invasive sensors to automate or assist in the control of proper anesthesia levels of a subject would be of great assistance. In a similar manner, simple, easy feedback for proper ventilation control from non-invasive, easily applied sensors in animal research applications would be very beneficial. Obviously, such advances would not be limited to animal research as non-invasive physiologic measurements can be very useful for human applications as well.
It is an object of the present invention to minimize the drawbacks of the existing systems and to provide medical devices and techniques for deriving cardiac and breathing parameters of a subject from extra-thoracic blood flow measurements and for controlling the ventilation levels and the anesthesia levels of a subject based upon said measurements.